Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Printed information displayed in these manners cannot be changed. Devices that allow for the modification of information, such as electrically updated displays, are often heavy and expensive. Information may also be applied to sheet materials via magnetically written areas, for example, to carry ticketing or financial information. Such magnetically written data, however, is not visible.
Media systems exist that maintain electronically changeable data without power. Such system can be electrophoretic (Eink), Gyricon, or polymer dispersed cholesteric materials. An example of such electronically updateable displays can be found in U.S. Pat. No. 3,600,060, which shows a device having a coated, then dried emulsion of cholesteric liquid crystals in aqueous gelatin to form a field-responsive, bistable display. U.S. Pat. No. 3,816,786 also discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing, and it is suggested that the light absorbing electrode be prepared from paints containing conductive material such as carbon.
Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047. A substrate carries a first conductive electrode, one or more layers of encapsulated liquid crystals, and a second electrode of electrically conductive ink. The conductive inks form a background for absorbing light, so that the information-bearing display areas appear dark in contrast to background non-display areas. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. Because the liquid crystal material is nematic liquid crystal, the display ceases to present an image when de-energized, that is, in the absence of a field. A first flexible substrate is patterned which is coated. A second pre-patterned substrate is bonded over the coating.
U.S. Pat. No. 5,251,048 discloses a light modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being electrically driven between a planar state, reflecting a specific visible wavelength of light, and a focal conic state, transmitting forward scattering light. Chiral nematic liquid crystals, also known as cholesteric liquid crystals, potentially in some circumstances have the capacity of maintaining one of multiple given states in the absence of an electric field. Black paint can be applied to the outer surface of a rear substrate to provide a light absorbing layer forming a non-changing background outside of a changeable display area defined by the intersection of segment lines and scanning lines. A first glass substrate is patterned. A second patterned glass substrate is fixable spaced from the first substrate. The cavity is filled with liquid crystal.
U.S. Pat. No. 6,394,870 discloses directly depositing opaque conductive ink in an image wise pattern by screen-printing. A conductor is printed directly over a polymer dispersed cholesteric material. Displays having such a configuration require a light absorbing backing. The invention creates the light absorber by printing second conductors formed by screen printable carbon in a resin matrix. Carbon absorbs visible light, but also absorbs ultraviolet radiation that can be used to cure ultraviolet responsive conductive formulations. If an ultraviolet cured silver ink were used, the reflection of the silver would create negligible contrast between the reflective planar and the transmissive focal conic states. The drying process for opaque conductive inks requires many minutes to cure the ink.
A photo-curable silver composition from Allied Photo Chemical is disclosed in U.S. Pat. No. 6,290,881, incorporated herein by reference. The composition is comprised of an ultraviolet light curable organic mixture, a photoinitiator, a silver powder, and a silver flake composition. The silver flake composition comprises at least 20% of the weight of the silver powder. The disclosed compositions may be used to produce silver-containing coatings on a variety of different substrates. However, this material is not disclosed for use in display devices, nor does it disclose methods to photo-cure the ink in display devices or on heat sensitive substrates. It would be useful for flexible, and or heat sensitive displays to be fabricated using simple, low cost processes. Such processes could include inexpensive, high speed methods for forming second conductors, such as in roll-to-roll processing equipment.
U.S. Ser. No. 10/847,188 discloses a dye that was coated over the polymer dispersed cholesteric liquid crystal and dried to form a complementary light absorbing layer. Second conductors were printed over the complementary light absorbing layer using a photosensitive silver filled polymer thick film ink, such as UVAG® 0010 resinous material from Allied Photochemical of Kimball, Mich. After printing the polymer thick film (PTF) ink on a panel of the polymer dispersed cholesteric liquid crystal and dye on a flexible support, the ink was exposed to ultraviolet radiation greater than 0.20 Joules/cm2 to form a durable and conductive surface. The heat sensitive liquid crystal would thermally transition at these cure energies unless there was a mechanism for eliminating or removing the infrared (IR) component of the spectrum of light emitted that is absorbed by the target UV curable material. The exposed ink reached its specified conductivity after approximately 24 hours after exposure to the UV light source. However, until the specified conductivity was reached, some of the ink material would transfer to the backside of panels stacked or wrapped over the cured ink surface. To facilitate a roll-to-roll manufacturing process, it would be desirable to be able to cure the ink in such a fashion as to avoid thermal transitioning of the liquid crystal and eliminate any propensity to transfer UV cured ink to the backside of the next lap while wound in a roll. Further, it is desirable to more thoroughly cure the UV ink for increased conductivity, hardness and adhesion to the substrate.
A curing unit and method of curing ink is disclosed in U.S. Pat. No. 5,216,820A. The invention relates generally to a curing apparatus for use in screen process printing, and more particularly to an apparatus for curing photo-initiated polymer-based inks applied to flat and three dimensional articles. The summary of the invention relates to a dual chambered cover assembly in which is disposed a reflector assembly containing a curing lamp and a means for exhausting air from the chambers of the cover. The invention further provides for heat reduction of the UV cure process by utilizing a cover assembly that consists of inner and outer covers that create an outer cooling chamber there between, and an inner cooling chamber between the inner cooling chamber and reflector assembly. The invention also includes a reflector assembly that is disposed within a cover that has at least one opening in its outer surface and defines an outer chamber. The unit further includes means for exhausting heated air from the curing chamber, and a means for drawing air though at least one opening provided in the cover into the outer chamber and exhausting the air from the outer chamber. The ends of the cover assembly form ducts, which communicate with the means for exhausting air from the chambers of the cover. The exhaust means draws external cooling air into the outer cooling chamber through openings in the outer cover and exhausts the air to cool the unit. However, this patent does not disclose use for display devices, such as a flexible display containing material that is heat sensitive. Further, there is no mechanism for eliminating or removing the IR component of the spectrum of light emitted that is absorbed by the target UV curable material. This IR component must be minimized to avoid thermal transitioning of heat sensitive substrates, such as cholesteric liquid crystal.
A paper from the SGIA journal from the second quarter 2004, titled “The process window of UV-cured inkjet printing”, suggests the significance of different wavelengths of UV light for depth and surface polymerization of UV sensitive inks or coatings. The paper suggests that most basic mercury bulbs containing additives emit energy in both long and shortwave UV, which is sufficient to provide both a depth and surface cure where there is an absence of particles or the deposited material to be cured is relatively thin. Depth of cure (long wave) is critical for proper ink adhesion to the substrate, and surface cure (short wave) is critical for no blocking of stacked or wound substrates. Experiments with single bulb UV cure configurations, for curing a UV sensitive silver bearing ink such as Allied Photochemical UVAG 0010©, have shown that a single bulb configuration does not produce an adequate cure that enables a roll-to-roll manufacturing process. To cure the ink in a roll-to-roll process, it would be advantageous to achieve a full cure where the ink on the substrate is able to be wound in a roll a short time after UV exposure without adhesive failure or blocking concerns.
When illuminating UV curable ink on opaque surfaces, such as on a PDLC coating containing a light absorbing layer between the PDLC and photo-curable ink, the ink does not fully cure to the extent that it can be wound up in a roll-to-roll manufacturing process, causing some ink transfer to the backside of sequentially wrapped layers. The physical properties of UV curable materials are affected by the radiation source used to cure them. Physical characteristics such as thickness of the curable material and presence of silver particles and flake, diffusivity and absorptivity require balancing of the wavelength parameters of the radiation source, as short wavelengths are known to work on the surface, while longer wavelengths penetrate more deeply into the material, as discussed in ‘The Process Window of US-Cured Inkjet Printing’, R. W. Stowe, SGIA Journal, Second Quarter 2004, pgs. 3-6. Insufficient exposure of the curable material results in insufficient cure, producing low resistivity, in the case of conductive materials, and blocking, that is, transfer of the curable material between rolls when the rolls are wound after processing.
Commercially available UV curing equipment, consisting of a model LC-6B conveyor assembly and two F300S UV curing lamp assemblies is shown in FIG. 5, enable a rapid cure of the silver bearing photo initiator ink in a panelized process, but is not conducive to a roll-to-roll process. One use for this tool allows for the user to expose substrates to two different UV spectrums, which are generated from two different type (doped) UV lamps. The advantage of using different spectrums allows for the user to achieve depth and surface activation of the photo initiators in the ink by utilizing long and short UV wavelengths. This process is familiar to those skilled in the art of light curing technologies. One disadvantage of this type system is that it also creates high levels of heat in the form of IR waves and hot air from the cooling of the lamps, which can be detrimental to heat sensitive substrates. Exposing an IR spectra onto a PDLC surface can cause thermal transition of the PDLC layer, and can also cause thermal-induced damage to the flexible substrate as well. In addition, the Fusion lamps are positioned at a distance from each other and illuminate different areas of the curable material, which results in non-uniform cure of material. In the case of small articles, the system is unable to expose the small areas to exposure by both light sources in a single pass, resulting in increased manufacturing complexity. Exposure to two different UV lamp spectra would require back and forth movement for curing closely adjacent panels. It would be useful to combine two different UV lamp spectra into one path to better facilitate roll-to-roll processing of the displays, especially if the roll-to-roll operation is based on intermittent motion resulting in the substrate pausing in a way that part of the printed area is exposed to both lamps, with the substrate under the gap between the two light sources exposed to only one light source. This situation will result in significant and possibly unacceptable curing variability. The most significant factor is that even a small time delay of the order of less than a second between the exposures results in a less rapid and thorough cure than the simultaneous exposure to the different spectra of two different lamps.
An illumination system for combining light from two separate light sources is disclosed in U.S. Pat. No. 6,341,876B1. The invention relates to illumination systems for producing a beam of light for illuminating a spatial light modulator, the spatial light modulator producing a spatially modulated beam of light, which may be projected on to a display screen. However, this invention does not disclose use in a UV curing chamber for curing photosensitive inks, nor does it disclose how the IR spectrum could be minimized to avoid thermal transition of a heat sensitive substrate.
U.S. Pat. No. 6,621,087 discloses a device for curing a UV coating on a substrate, in particular on heat-sensitive materials, with at least one light source that is located above the substrate. The light can be directed to the UV coating via a reflector system, including at least one barrier, which prevents the direct beam path of the light source from striking the substrate. The UV radiation emitted by the light source is reflected by a UV reflection coating of the barrier through the light source to the reflectors located behind the light source, and the barrier includes at least one heat absorbing body that absorbs the heat radiation emitted by the light source. In one embodiment of the invention, it is possible to focus the UV radiation on the substrate. This invention enables an effective separation of the UV radiation from the IR radiation in order to reduce the heat load of the substrate and at the same time to achieve a high UV intensity through short beam paths necessary for curing. However, U.S. Pat. No. 6,621,087 does not involve combination of multiple light sources into a single path to achieve improved cure and ease/improvements in roll-to-roll or discrete parts manufacturing.